The present invention pertains to a process for separating isometric dichlorotoluenes by adsorption on certain zeolites.
Dichlorotoluenes are generally prepared by chlorinating toluene or monochlorotoluenes in the presence of a Lewis acid such as the trichlorides of aluminum, iron or antimony alone or combined with a cocatalyst, e.g., sulfur or sulfur chlorides.
The chlorination reaction leads to the formation of mixtures containing dichloro -2,4, -2,5, -2,6, -3,4 and -2,3 toluenes. These mixtures can be separated by distillation of the other chlorinated products of toluene or monochlorotoluenes (monochlorotoluene and trichlorotoluenes). The five above-mentioned isomers are obtained in varying proportions depending on the starting products used in the chlorination reaction.
The dichlorotoluene mixtures can also be separated by distillation into two fractions boiling at ca. 201.degree. C. and ca. 209.degree. C. The first fraction comprises the -2,6, -2,4 and -2,5 isomers, and the second fraction consists of the -3,4 and -2,3 isomers.
It has also been generally admitted that it is not possible to obtain all the different isomers in the pure state under economically acceptable conditions according to the conventional distillation or fractionated crystallization techniques. The distillation does not permit, in particular, separating the constituents of the two fractions boiling at ca. 201.degree. C. and ca. 209.degree. C. due to the very small difference between the boiling points of the isomers. Only the dichloro-2,3-toluene can be separated by distillation, provided that o-chlorotoluene is used as the starting product. As far as fractionated crystallization is concerned, it is not generally suitable due to the existence of numerous eutectic mixtures.
In view of the above considerations, other separation techniques have been proposed. In particular, U.S. Pat. No. 4,254,062 discloses a process for separating the isomeric dichlorotoluenes by means of type X or Y zeolites.